1. Field of the Invention
The present invention is related generally to an electro-acoustic transducer in which a circular transducer plate is arranged in a transducer housing, clamped between bearing, or support, members at its edge region and is provided with a piezo-electric layer.
2. Description of the Related Art
During the manufacture of electro-acoustic transducers, one of the goals to be achieved is to exercised care that the relationship between the acoustic specification factors of the acoustic field and the electrical quantities of the transducer are largely frequency independent in the transmission range.
The frequency dependency of the relationship between the specification factors of the acoustic field and the electrical quantities of the transducer is particularly defined by the frequency dependency of the oscillatory, mechanical structure composed of the membrane and the coupled air chambers or the like.
The membranes of high-grade acoustic receivers of, for example, capacitor microphones, are clamped and arranged so that the resonant frequency corresponding to their fundamental oscillation lies above the frequency range of the interest, i.e. outside the range in which they are to be used. This is so that the relationship between the movement of the membrane and the specification factors of the acoustic field is practically frequency independent in this frequency range.
In electro-acoustic transducers as used in the telephone industry, by contrast, it is usually not possible for reasons of efficiency to select the self-resonances of the membrane to lie outside the frequency range of interest. In order to nevertheless reduce the frequency dependency of the electro-acoustical transmission factor, it is standard practice to equip such transducers with correspondingly tuned resonators with whose assistance resonance peaks are compensated.
Instead of the usual membrane, recent piezo-electric transducers are formed of a transducer plate clamped at its edge region between two mounting members. The transducer plate is provided with a piezo-electric layer. When such plate is electrically or acoustically excited, then pronounced exaggerations, or distortions, are formed in the plate dependent on the measured acoustic pressure and on the frequency. Such distortions, which are distinguished by circular nodal lines and nodal diameters, may be made visible with holographic interferometry.
For cylindrically symmetrical transducer plates, the distortions distinguished by nodal diameters play no part. The circular nodal lines, however, are critical. Thus, the natural frequencies of a transducer having a circular transducer plate clamped at its edges between support members can, for example, be as follows:
Fundamental Resonance (.sigma.=0, h=0)--approximately 1 to 1.5 kHz. PA0 First Circular Nodal Line (.sigma.=1, h=0)--about 4 kHz. PA0 Second Circular Nodal Line (.sigma.=2, h=0)--about 7 to 9 kHz. PA0 Third Circular Nodal Line (.sigma.=3, h=0)--about 14 kHz, whereby .sigma. denotes the number of circular nodal lines and h denotes the number of nodal diameters.
As already described, the resonant peaks must be attenuated so that tolerance ranges described by individual telephone administrations are not transgressed. For example, it is known to attenuate the fundamental resonance by about 15 dB with a Helmholtz resonator. (See, for example, Siemens Zeitschrift, Vol. 46, April 1972, No. 4, pages 207-209).
The partial oscillation characterized by the first circular nodal line can be attenuated by two half-wave resonators, as in German Patent No. 1,167,897.
The partial oscillation characterized by the second circular nodal line was previously not attenuated since it did not fall within the tolerance pattern prescribed by the telephone administrations. Due to the expansion of the tolerance ranges from 8 kHz to 10 kHz, however, this partial oscillation leads to a transgression of the tolerance range and so must be attenuated.
An attenuation of this partial oscillation can be carried out with a Helmholtz resonator having a broadband effect that, however, is difficult to arrange in the existing transducer housing.